Automotive accessory drives utilize rubber/textile cord belts in order to transfer a portion of the available crankshaft torque to the driven accessories. The belt length is usually quite long as most accessories are usually driven with single belt.
Moreover, the engine does not produce a constant torque output but produces continuously varying torque (impulses) particularly at low speeds (idle) and heavy accessory loads. This varying torque results in roughness which is increasing as new engines are designed for more power and efficiency.
Furthermore, additional new devices, such as "dual mass" fly wheels, are being added to the powertrain increasing roughness at crankshaft speeds. The combined inertia of the accessories (alternator, cooling fan, air conditioner compressor, power steering pump, etc.) is also increasing, increasing the rate at which an engine accelerates and decelerates.
Three distinct problems are created particularly with newer engine designs. First, the long elastic belt driven by the pulsating torque of the crankshaft and in turn driving the heavy inertias of the accessories often results in an angular resonance that amplifies the tension fluctuations in the belt causing noise, vibration, tensioner wear and increased peak bearing and structural loads.
Secondly, the engine must pass through the resonant frequency momentarily when starting up or shutting down, even when a convention decoupler is used. The engine roughness is much worse at the low start up and shut down speeds. This roughness, acting at a relatively low resonant frequency, can cause large momentary tension reversals in the belt drive. A conventional belt tensioner does not have sufficient tension to cope with the reverse mode. Further, tension reversal causes the tensioner to move violently, momentarily disabling it. The result is belt squeak and occasional tensioner damage.
Thirdly, a combined load from the accessories load and their inertias will sometimes cause the belt to slip momentarily during acceleration until the peak acceleration subsides. Correcting this with belt tension or drive size is often undesirable or not possible.
Crankshaft decouplers are well known to the industry for addressing resonance problems. Crankshaft decouplers have generally all utilized rubber springs which have generally shown marginal durability. The rubber component is exposed to a harsh environment which causes the rubber to degrade which results in a loss of durability. Moreover, the performance of decouplers incorporating rubber rings must be compromised in order to obtain reliability.
Torque limiting clutches are utilized to address torque overload problems. Such clutches are described in U.S. Pat. No. 3,618,730 which utilize a first torsion spring to provide a positive connection between the driving hub and a cage and a second torsion spring to provide a circumferential frictional connection between the cage and a driven pulley. As the torque increases, the driving hub will angularly displace the cage until the second torsion spring winds down causing the second torsion spring to decrease in diameter releasing the circumferential engagement. After the maximum torque has been exceeded, the second torsion spring will no longer engage the driven pulley allowing it to slip.
Previous devices normally rely upon a wrap spring to limit torque. These devices have been found to be insufficiently reliable in practice.
Wrap spring, roller/sprag clutches and single band clutches have been used as one way devices. A wrap spring or roller ramp/sprag clutch all engage in a harsh, high shock manner which increases stress and reduces reliability. A single band clutch does not provide sufficient drive capability at low overrunning torque.